Molecular Spectroscopy Applications Research Articles

Selected latest applications of molecular spectroscopy in natural product analysis

The field of molecular vibrational spectroscopy applied to natural products is advancing extremely fast. Traditionally applied separation techniques (LC-, μ-LC–MS, GC, CE-MS) offer the advantages of high selectivity/sensitivity, but their application for routine quality control is limited due to long analyses times. Therefore, molecular spectroscopy in combination with multivariate analysis (MVA) enjoys excellent reputation, because of the fast and non-invasive measurement enabling the analysis of several physical and chemical parameters simultaneously. Near infrared (NIR; 4.000–10.000cm−1), attenuated total reflection (ATR; 400–4.000cm−1), Raman and far ultraviolet (FUV;120–200nm) spectroscopy have permanently increased their efficiencies for quality control of predominantly food stuff, but also of other natural products including mainly medicinal plants. All four techniques enable not only a quantitative analysis of potent ingredients, but also qualitative fingerprint analysis for the discrimination of, e.g., species and/or geographic origin, respectively. Thereby, each individual spectroscopic technique possesses its specific strength. Powerful miniaturized portable spectrometers based on linear variable tuneable filter (LVTF) or micro-electro-mechanical systems (MEMS) are helpful in order to prevent consumers from deception on one hand, on the other hand they represent powerful analytical instruments for measurements in the field. 2-Dimensional correlation spectroscopy (2DCOS) represents a powerful technique for monitoring the dynamics of a system including temperature stability, extraction procedures etc. Imaging and mapping spectroscopy using infrared radiation and/or Raman scattering are not only suitable for classification of food stuff including e.g. maize kernels and/or coffee beans, but also for localizing the distribution of ingredients down to a resolution of 4μm.In the present contribution, the latest progresses of the different techniques are introduced and their applicability in the fields of natural product analysis will be discussed in detail by distinct selected applications.

Frequency Tuning and Modulation of a Quantum Cascade Laser with an Integrated Resistive Heater

We present a detailed experimental investigation of the use of a novel actuator for frequency tuning and modulation in a quantum cascade laser (QCL) based on a resistive integrated heater (IH) placed close to the active region. This new actuator is attractive for molecular spectroscopy applications as it enables fast tuning of the QCL wavelength with a minor influence on the optical output power, and is electrically-controlled. Using a spectroscopic setup comprising a low-pressure gas cell, we measured the tuning and modulation properties of a QCL emitting at 7.8 μm as a function of the active region and IH currents. We show that a current step applied to the IH enables the laser frequency to be switched by 500 MHz in a few milliseconds, as fast as for a step of the current in the active region, and limited by heat dissipation towards the laser sub-mount. The QCL optical frequency can be modulated up to ~100 kHz with the IH current, which is one order of magnitude slower than for the QCL current, but sufficient for many spectroscopic applications. We discuss the experimental results using a thermal model of the heat transfer in terms of cascaded low-pass filters and extract the respective cut-off frequencies. Finally, we present a proof-of-principle experiment of wavelength modulation spectroscopy of a N2O transition performed with a modulation of the IH current and show some potential benefits in comparison to QCL current modulation, which results from the reduced associated amplitude modulation.

Mode-locked mid-infrared frequency combs in a silicon microresonator

Mid-infrared (mid-IR) frequency combs have broad applications in molecular spectroscopy and chemical/biological sensing. Recently developed microresonator-based combs in this wavelength regime could enable portable and robust devices using a single-frequency pump field. Here, we demonstrate a mode-locked microresonator-based frequency comb in the mid-IR spanning 2.4–4.3 μm. We observe high pump-to-comb conversion efficiency, in which 40% of the pump power is converted to the output comb power. Utilizing an integrated PIN structure allows for tuning the silicon microresonator and controlling cavity soliton formation via free-carrier detection and control. Our results significantly advance microresonator-based comb technology toward a portable and robust mid-IR spectroscopic device that operates at low pump powers.

Subterawatt few-cycle mid-infrared pulses from a single filament

Filamentation-assisted pulse compression in the gas phase is shown to enable the generation of subterawatt few-cycle pulses in the mid-infrared (mid-IR). With both spatial modulation instabilities and excessive plasma scattering of the mid-IR beam prevented through a careful choice of gas pressure and input peak power, providing a single-filament regime of pulse propagation, peak powers as high as 0.3 TW are achieved in a truly single-mode, almost diffraction-limited 35 fs output at a central wavelength of 4 μm. Applications in molecular spectroscopy, semiconductor electronics, high-field physics, standoff detection, and innovative x-ray sources are envisaged.

Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy.

Nonlinear optical frequency conversion is one of the most versatile methods to generate wavelength-tunable laser light in the mid-infrared region. This spectral region is particularly important for trace gas detection and other applications of molecular spectroscopy, because it accommodates the fundamental vibrational bands of several interesting molecules. In this article, we review the progress of the most significant nonlinear optics instruments for widely tunable, high-resolution mid-infrared spectroscopy: continuous-wave optical parametric oscillators and difference frequency generators. We extend our discussion to mid-infrared optical frequency combs, which are becoming increasingly important spectroscopic tools, owing to their capability of highly sensitive and selective parallel detection of several molecular species. To illustrate the potential and limitations of mid-infrared sources based on nonlinear optics, we also review typical uses of these instruments in both applied and fundamental spectroscopy.

Congratulations Larry!

It is with great pleasure that I compose this editorial in celebration of our Editor in Chief's 2013 Pittsburg Spectroscopy Award. If this Editorial goes to plan, this will be the only article in recent years not to have been vetted by our illustrious Editor in Chief, so Larry, I apologize for circumventing your solid Editorial system but…SURPRISE! The 2013 Pittsburg Spectroscopy Award is presented by the Spectroscopy Society of Pittsburgh and presented in honor of an individual who has made outstanding contributions in the field of spectroscopy. Since before I was born, a younger then ‘Dr.’ Nafie was shaking things up in the world of vibrational spectroscopy; his 1972 paper Reorientation and vibrational relaxation as line broadening factors in vibrational spectroscopy has received a remarkable 228 citations to date. Just one of the many classic articles from Larry's early career. Since these extraordinary beginnings, Larry has amassed a bibliography of 274 articles from 1969 to 2012 (as indexed by the Web of Science) having attracted an impressive 341 citations in 2011 alone. Plus, of course, there is his infamous book, Vibrational Optical Activity – Principles and Applications. Larry is perhaps the world's most foremost expert on vibrational circular dichroism (VCD) and Raman optical activity (ROA). He has pioneered the use of these spectroscopic techniques for addressing questions of structure and function for a wide range of biological or model biological molecular systems. Most recently, demonstrating their use, combined with state of the art computations, for investigations of protein structure function relationships. In part, Larry's recognized success and impact derives from his in-depth theoretical knowledge and understanding of matter–radiation interactions coupled with his experimental prowess in the clever design and implementation of instrumentation. In addition, he has identified important problems in biophysics where these novel spectroscopic methodologies can be applied. It is accurate to say that Larry has built both the theoretical as well as instrumental machinery that the current fields of VCD and ROA are built on. Larry's scientific impact has been further enhanced by his efforts to make state-of-the-art instrumentation required for these circularly polarized measurements commercially accessible for laboratories throughout the world via Biotools, Inc., the company he founded offering VCD and ROA instrumental platforms. This recognition by the Pittsburgh Conference is not the first award that recognizes his creative and significant scientific achievements. Larry's previous awards in recognition of his innovative contributions to the modern techniques and current applications of molecular spectroscopy including an Alfred P. Sloan Fellowship, the Coblentz Award, the Bomem Michelson Award, the Meggars Award, and he is a Fellow of the Society of Applied Spectroscopy. Thank you Larry and congratulations!

Recent Analytical Applications of Molecular Spectroscopy in Bioorganometallic Chemistry—Part I: Metal Carbonyls

: This is the first part of a two-part review on the analytical applications of molecular spectroscopy in bioorganometallic chemistry since 2005. In this case, radiopharmaceutical studies are included and the review is focused particularly on biological molecules labeled with metal carbonyl fragments.

Computation of AlO B 2Σ + → X 2Σ + emission spectra

Abstract Application of molecular spectroscopy to analytical chemistry usually requires accurate description of the particular transition of interest. In this communication we describe the creation of a list of spectral lines. Following the introduction and definition of the line strength, we present a recipe for computation of diatomic-line-strengths, including the Hönl–London factor and electric dipole line strength for each spectral line. The diatomic eigenfunction is discussed including Hund’s case basis functions. In our data tables we prefer use of Hund’s case (a) basis, and we apply the usual Born–Oppenheimer approximation for the electronic-vibrational strengths. This allows us to generate the table of line strengths that we frequently apply for spectroscopic temperature determination. Using these line-strength tables, we present theoretical AlO emission spectra for the B–X system of AlO. These emission spectra are computed for temperatures of 3000 and 6000 K and for typical spectroscopic resolution used in laser-induced optical breakdown studies.

Interfaced SEM/EDX and micro-Raman Spectrometry for characterisation of heterogeneous environmental particles — Fundamental and practical challenges

The molecular character of atmospheric particulate matter is of prime importance when interpreting air pollution trends and its subsequent influence on environmental monitoring and preventative conservation. The known methods of estimating the molecular composition normally involve elemental analysis of particles (both as bulk and computer controlled analyses of single particles) with subsequent multivariate analyses to clusterise the elements in groups of elements that are closely related to each other. With this approach one can at best suggest associations. Evidently the application of molecular spectroscopy in addition to elemental concentration profiles would provide intimate information regarding the nature of the particles and consequently their fate. This paper gives an overview of research performed in our laboratory and describes the optimisation of experimental parameters to use scanning electron microscopy with energy-dispersive X-ray detection (SEM/EDX) or electron probe X-ray microanalysis (EPXMA) in parallel with micro-Raman Spectrometry (MRS) to investigate single environmental particles. The challenges associated with the two stand-alone techniques are revealed and consequently those posed with an interfaced approach are discussed. Preliminary results, of an initial investigation of the SEM/EDX interfaced with MRS to ultra-fine heterogeneous environmental particles, are given.

Remarks on dynamical and noncompact groups in physics and chemistry

In contrast to the usual degeneracy or symmetry groups, the new dynamical groups generate all the energy levels and spectrum of a system in one infinite-dimensional representation. The origin and nature of these groups as they came up first in high-energy physics, then in atomic physics, are outlined. The known dynamical groups, e.g., for the rigid rotor, the harmonic oscillator, and the hydrogen atom, are displayed. Possible applications in molecular spectroscopy are discussed.

On the applicability of centroid and ring polymer path integral molecular dynamics for vibrational spectroscopy

Centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD) are two conceptually distinct extensions of path integral molecular dynamics that are able to generate approximate quantum dynamics of complex molecular systems. Both methods can be used to compute quasiclassical time correlation functions which have direct application in molecular spectroscopy; in particular, to infrared spectroscopy via dipole autocorrelation functions. The performance of both methods for computing vibrational spectra of several simple but representative molecular model systems is investigated systematically as a function of temperature and isotopic substitution. In this context both CMD and RPMD feature intrinsic problems which are quantified and investigated in detail. Based on the obtained results guidelines for using CMD and RPMD to compute infrared spectra of molecular systems are provided.

Semi-Parametric Estimation in the Compositional Modeling of Multicomponent Systems from Raman Spectroscopic Data

Identification and quantification of molecular species are central applications of molecular spectroscopy. In complex multicomponent systems like tissue samples, linear parametric models are often used to estimate the relative concentrations of the biochemical components of the sample. In situations where not all of the components of the sample are known or modeled, such parametric models can suffer from omitted variable bias and result in skewed estimates of component concentrations. We propose a semi-parametric approach that tries to avoid this omitted variable bias by effectively including unknown covariates as a non-parametric term in the regression equation. Constituent concentrations estimated with such partial linear models should outperform strict parametric linear models when the user has limited information on the composition of a multi-constituent system.

IV-VI Semiconductor Mid-IR Lasers

ABSTRACTMid-IR lasers fabricated from narrow bandgap IV-VI semiconductors are proven devices for molecular spectroscopy applications. Wide single-mode tunability, low waste heat generation, and large spectral coverage from about 1000 cm−1 (10 µm) to about 2500 cm−1 (4 µm) have allowed development of laser absorption spectroscopy instrumentation for fast and sensitive measurement of specific gas phase molecules. For example, IV-VI mid-IR lasers with emission in the 5.2 µm spectral range have recently enabled the development of breath analysis instruments for real-time measurement of exhaled nitric oxide (eNO). Laser tunability with current ramping is sufficient for simultaneous measurement of exhaled carbon dioxide (eCO2), a capability that allows highly accurate determination of eNO concentrations in the low ppb range, a sensitivity required for assessing airway inflammation in patients with asthma. After discussing emerging medical diagnostic applications this paper reviews recent progress in the development of liquid-nitrogen-free cryogenic cooling systems for IV-VI mid-IR lasers. A description of continuing research on the development of improved IV-VI lasers, where the primary objective is to fabricate devices with continuous wave (cw) operation at room temperature is then presented. Theoretical and experimental analysis of (111)-oriented multiple quantum well (MQW) IV-VI materials show that it should be possible to reduce lasing thresholds significantly. In addition, results from transferring IV-VI materials from low thermal conductivity growth substrates to higher thermal conductivity copper show that new laser packaging methods can significantly improve active region heat dissipation. Together, these new materials and device packaging methods promise to enable the fabrication of IV-VI mid-IR lasers with cw operation at room temperature.

Application of Molecular Spectroscopy in the Mid-Infrared Region to the Determination of Glucose and Cholesterol in Whole Blood and in Blood Serum

The feasibility of applying molecular spectroscopy in the mid-infrared region to the quantitation of glucose and cholesterol in whole blood and in blood serum was examined by measuring the samples in the dry state on a polyethylene carrier. The partial least-squares (PLS) algorithm was employed to set up the calibration models. Analysis was performed by normalizing the spectra to a standard area. Evidence that this approach is warranted was obtained through experiments where potassium thiocyanate served as the internal standard. The following average PRESS1/2 values (PRESS stands for prediction residual error sum of squares) emerged from the calibration: glucose, 22.3 mg/dL; cholesterol, 31.3 mg/dL. Such errors are comparable with those achieved in attenuated total reflectance (ATR) measurements. The calibration models set up for blood serum samples fit better than those for whole blood samples. If the procedure of sample application onto the carrier is improved, the method may work well for the determination of glucose and cholesterol in whole blood and in blood serum.

Line shapes in coherent ion dip spectroscopy of polyatomic molecules.

Using two narrow-band Fourier-transform limited, tunable ns laser pulses the observation of coherent adiabatic Raman processes becomes feasible for polyatomic molecules. Recent experiments reveal the possibility of an effective suppression of ionization in an ion dip experiment using a lambda-type resonant coherent optical-optical UV-double-resonance process and suggest its application in molecular spectroscopy. The experiments, including the observed ionization enhancement for off-resonance conditions, are quantitatively described using a few-level density matrix approach.

Structure-property relation in spectroscopy

Structure elucidation is the most important application of molecular spectroscopy. Nowadays the computer enables the chemists to derive more of the desired information from the recorded spectra by exploiting structure property relationships. Sophisticated methods of computational chemistry, on-line access to large databases, and expert systems are those disciplins, which are important for progress in spectroscopy.

Low-temperature spectroscopy of organic molecules in solids by photochemical hole burning

Characteristic features of photochemical hole-burning (PHB) in the impurity spectra of low-temperature solids and PHB applications in molecular spectroscopy are considered. The evolution of a no-phonon hole and a phonon sidehole in excitation and fluorescence spectra is analysed on the basis of model calculations. Some more complex models for PHB are considered, which take into account reverse reactions, the optical thickness of the sample, the inhomogeneous dispersion of both homogeneous linewidths and transition energies of the photoproduct and quasi-static impurity-impurity interactions. The effects of PHB on fluorescence line narrowing are discussed. By PHB homogeneous linewidths of purely electronic and vibronic no-phonon lines in the spectra of some porphine and phthalocyanine derivatives in various matrices are measured and their temperature dependence is studied. The latter is found to be essentially different in polycrystalline and glassy matrices. Line-broadening mechanisms are discussed. By PHB the existence of an inhomogeneous distribution of vibrational frequencies in molecular impurities is established. The applications of PHB in the studies of photochemical reactions in solid solutions of phthalocyanine derivatives and tetracene are regarded and the mechanisms of these reactions are discussed. The results obtained by PHB for chlorophyll and its derivatives are presented.

Applications of molecular spectroscopy to relationships between biological activity and structure.

Applications of molecular spectroscopy to relationships between biological activity and structure.

New Techniques and Applications in Molecular Spectroscopy

Molecular Spectroscopy Edited by George Sell. (Report of a Conference organized by the Spectroscopic Panel of the Hydrocarbon Research Group of the Institute of Petroleum, and held in London, 28–29 October 1954.) Pp. viii + 234. (London: Institute of Petroleum, 1955.) 42s. net.